Increasing flood risks create a major political and institutional challenge for the world’s coastal cities, as ambitious and proactive action at the local level over the next decades will be needed to avoid large-scale flood disasters. However, the implementation of flood risk management policies meets many obstacles.

In a recent study written with colleagues Colin Green, Robert Nicholls and Jan Corfee-Morlot as part of an OECD project on urban vulnerability, we estimate how flood risks could change in the future in 136 coastal cities, in response to increasing population and wealth, local environmental change, and climate change. We find that because current flood defenses and urbanization patterns have been designed for past environmental conditions, even a moderate change in sea level is sufficient to make them inadequate, thus magnifying flood losses to catastrophic levels. If no action is taken to reduce flood vulnerability, most coastal cities would become inhospitable and dangerous places to live, with annual losses in excess of US$1 trillion dollars.

What can be done?

Our analysis suggests that upgrading defenses could mitigate these losses and the impacts of rising sea levels. However, these upgrades need to include a package of risk management policies. First, coastal defenses should make use of the protection the environment can offer for free: Marshes, seagrass beds, coastal and kelp forests, and coral reefs provide natural buffers that absorb the energy from waves and storms, making it easier and cheaper to protect urban development. In addition artificial constructions are also required to provide full protection. We are not only talking about dykes: a city surrounded by dykes will need pumping systems to drain rainfall water, and a harbor will need moving barriers to let ships in and out of the port.